Conventional confocal optical microscopes are known for their extremely short depth of focus and improved transverse resolution. The main drawback with conventional confocal scanning optical microscopes is that they illuminate only one point on the object at a time. To scan the object either the sample or beam must be mechanically moved to form a raster image of the object. Mechanical scanning is time consuming.
In a paper appearing in The Journal of the Optical Society of America, Vol.58, No.5, May 1968, there is described a tandem-scanning reflected-light microscope. In the described microscope both the object plane and the image plane are scanned in tandem so that only light reflected from the object plane is included in the image. The object to be viewed is illuminated with light passing through a plurality of holes in a rotating disc which are focused onto the object by an objective lens. Thus, a large number of points on the object, corresponding to the holes which are illuminated, are illuminated at one time. The holes themselves are located along spiral paths. By rotating the disc raster scan of the object is obtained. Reflected light from the illuminated spots is directed to the opposite side of the same apertured disc and passes through optically congruent holes on diametrically opposite sides of the rotating disc. The image obtained had better contrast and sharpness than possible with the usual reflected-light microscope. This is due to the fact that reflected light from within the microscope and other stray light coming from other points is intercepted by the opaque portions of the disc and the image only comprised reflection from the illuminated spots. The system of the prior art is extremely complex and difficult to align because of the necessity of assuring that the light reflected from the illuminated spots must pass through congruent holes on the opposite side of the rotating disc. Therefore, despite its obvious advantages, only two or three microscopes of this type have been constructed in the last 20 years.
The present invention employs the principals of a confocal optical microscope such as illustrated schematically in FIG. 1. Light input from a laser, arc lamp or other light source impinges upon a plate 11 which includes a pinhole 12. The light travels through the pinhole to the objective lens 13 and is focused on the object plane 14. Light reflected from the object plane travels back through the pinhole as shown and can be viewed. Reflection from out of focus planes such as plane 16 does not converge at the pinhole and is therefore blocked by the plate 11. In operation the reflected light passing through the pinhole impinges upon an associated detector The detector output is maximum when the object is located at the focus of the lens, otherwise the light received at the pinhole is defocused and the amplitude of the signal falls off rapidly as the position of the object is changed on either side of the object plane. The system has a very short depth of focus in addition to excellent transverse definition.
As previously described, such a microscope can be mechanically scanned to form a raster image by moving the object or pinhole in a raster pattern.